In the field of modern shuttleless textile looms operating at high speed, it is well known that the effective clamping of the weft yarn by the weft propelling grippers is one of the problems most difficult to solve in that considerable difficulties are present due to technical requirements which are of opposite character.
In this respect, because weft yarn generally does not have a very great strength, it is required to enter smoothly at low tension between the gripping members of the gripper so that it does not undergo damage.
On the other hand, in order to prevent the weft yarn from being able to withdraw from the clamping members of the gripper because of either the tension necessary for unwinding the weft yarn from the reel or the tension induced by the necessary weft braking devices, or because of tearing, vibration or intense acceleration, it is necessary for said clamping members of the gripper to retain the weft reliably by clamping it strongly. In addition, account must be taken of the fact that the grippers slide along the loom sley which in its turn rotates in a direction orthogonal to its length. Thus from the time the inserting gripper grips the weft yarn which is stretched between the reel and the edge of the fabric under formation until the time the yarn is cut by the edge of said fabric, said weft yarn must be able to slide easily between the gripping members of the inserting gripper in the direction from the reel to the fabric edge, in order to enable the gripper to travel through its curved path without tearing the weft yarn which is still fixed to the edge of the fabric. Thus, in order to enable the insertion of the weft yarn into the inserting gripper, the retention of said yarn during its transportation, and the transfer of the weft yarn from the inserting gripper to the withdrawing gripper at the centre of the shed to be effected in an optimum manner, the weft yarn must be able to slide easily between the gripping and clamping members in the longitudinal direction of the inserting gripper. In the transverse direction it must be able to slide easily only in the unwinding direction from the reel, whereas in the other direction, however, the weft yarm must be prefectly blocked. Methods are already known in the state of the art for gripping and clamping the weft yarn.
One of said known methods uses positively controlled gripping and clamping members, i.e. members which are opened by suitable external control devices in order to receive or release the weft yarn, whereas strong springs provide for keeping the members firmly clamped together during the other stages. Although such members perfectly satisfy the requirement of gripping and releasing the yarn smoothly, and of retaining it firmly during its transportation, they have a series of considerable drawbacks due substantially to the enormous difficulty of operating the gripping and clamping members in phase with the external control devices, especially in the change-over of the yarn between the weft inserting and withdrawing grippers at the centre of the shed, because of the high loom speed and in particular because of the fact that slack inevitably arises between the various mechanical members. In addition to the greater constructional complexity and the consequent greater cost, in these types of member there is also excessive wear and a greater possibility of fracture because they comprise elements which are in contact with each other at the high speed necessary for the drive movements.
According to a further known method, gripping and clamping members are used which are kept constantly closed by suitably set springs which are adjustable. In this case, the closure of the gripping members is obviously a compromise between the requirement to grip and release the weft yarn smoothly, and the requirement to retain it firmly during its transportation. Special accurate controls are therefore necessary in order to attain acceptable operation, and sometimes this latter is even impossible to attain because of the characteristics of particular irregular heavy yarns such as those used for furnishing fabrics which, requiring high tension for unwinding them from the reel, have to be energetically clamped by the gripping members of the weft inserting and withdrawing grippers.